Light emitting diode (LED) is a kind of semiconductor device that can exchange signals or be used as a light source by converting electric signal into infrared rays or light using the characteristics of compound semiconductors.
The LED generating highly efficient light with low voltage has excellent energy saving effect. In recent years, as the brightness problem of the LED, which was a limit of the LED, has been highly improved, the LED is being widely used throughout industry as a whole such as backlight units, electric display boards, indicators, electric home appliances, and various automated machines.
For example, the emission spectrum of a gallium nitride (GaN)-based LED widely ranges from ultraviolet rays to infrared rays, and does not contain environmentally harmful substances such as arsenic (As) and mercury (Hg). Therefore, it is getting highly recognized in the environment-friendly aspect.
As an example of the light emitting diode, the nitride-based light emitting diode has a structure, wherein a buffer layer on the top of a sapphire substrate, a n-type nitride semiconductor layer, an active layer and a p-type nitride semiconductor layer are laminated in order; a part of the n-type nitride semiconductor layer is exposed from the p-type nitride semiconductor layer to the part of the n-type nitride semiconductor layer by mesa-etching; an n-electrode is formed on the top of the exposed n-type nitride semiconductor layer; a transparent electrode is formed on the top of the p-type nitride semiconductor layer and a p-electrode is formed on the top of the transparent electrode.
When applying a voltage to the p-electrode and the n-electrode, holes and electrons flow from the p-type nitride semiconductor layer and the n-type nitride semiconductor layer into an active layer, and recombined on the active layer, resulting in light emitting
Problems on improving efficiency of the light emitting diode may reside in: (1) efficiency reduction due to remaining carriers, which are not formed into excitons when the ratio of the hole and the electron are out of the ratio of 1:1 for generating one photon (at the time of bad charge balance), (2) non-radioactive recombination of excitons, and (3) light trapping phenomenon, which is that the light generated in an device does not escape out of the device due to a large refractive index difference between air outside the device and structural materials of the device.
On the other hand, the LED is a device emitting light when the inserted current carriers are recombined at p-n junction or change energy level, but a photo diode is a device using phenomenon, which generates a current or voltage by carriers generated after light irradiation on the p-n junction. Namely, photodiodes are similar to LEDs, but have inverse functions. Photodiodes convert light energy into electric energy, but LEDs convert electric energy into light energy. Photodiodes have characteristics of rapid response speed, wide sensitivity wavelength and good straightness of photocurrent.
In a solar cell device using photodiodes, materials making up an active layer absorb light from outside, thereby producing excitons. The excitons are separated into holes and electrons at the interface between p-type and n-type organic or inorganic semiconductor materials by an electric field applied to the solar cell device, and light can be converted into electricity by collecting the carriers at the electrodes of both ends of the device. Accordingly, the more light is absorbed at the active layer, the more electricity can be produced. But it is difficult to absorb large amount of light due to a very thin active layer in a thin film solar cell. On the contrary, if the active layer is too thick, the excitons formed by light absorbing are recombined and disappeared before reaching the interface of the p-n materials. Accordingly, the excitons cannot be separated into carriers, which are actually used.
Accordingly, structure, which can absorb light on a thin film solar cell as much as possible and efficiently separate the produced excitons as a carrier at the same time, is needed. Further, light trapping, which make the light irradiated to the front side of the solar cell enter the solar cell device without light reflection, and in the rear side, make the light inside the device stay in the device as long as possible without escaping out of the device, should be induced.
In order to solve the above problems, in particular, for improving LED efficiency, a method for controlling energy band gap by adding various material layers for adjusting charge balance, a method for various surface-treatment to LED such as micro lens or surface texturing for reducing the light trapping phenomenon LED, and a method for reducing light trapping by causing many scattered reflection in a device by using plasmon phenomenon have been tried (see Korean Patent Publication No. 10-2010-0068777, Korean Patent Publication No. 10-2012-0038472 and the like).
Further, in order to enhance efficiency of the solar cell, there are studies for improving efficiency of separating exciton as a carrier by largely expanding the interface between a hole transfer layer and an electron transfer layer through bulk heterojunction (BHJ) structure, and a method for controlling an energy band gap by controlling materials. In particular, it is tried to improve hole mobility or electron mobility of an organic solar cell by synthesizing a novel organic material.